Transducers generally convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. Acoustic transducers, in particular, convert electrical signals to acoustic waves and acoustic waves to electrical signals using inverse and direct piezoelectric effects. Acoustic transducers generally include acoustic resonators, such as bulk acoustic wave (BAW) resonators and surface acoustic wave (SAW) resonators and may be used in a wide variety of electronic applications, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and other portable communications devices. For example, BAW resonators may be used for electrical filters and voltage transformers. Generally, an acoustic resonator has a layer of piezoelectric material between two conductive plates (electrodes), which may be formed on an acoustic reflector. BAW resonator devices, in particular, generate acoustic waves that can propagate in lateral directions when stimulated by an applied time-varying electric field, as well as higher order harmonic mixing products. The laterally propagating modes and the higher order harmonic mixing products may have a deleterious impact on functionality.
What is needed, therefore, is a device useful in mitigating acoustic losses at the boundaries of the BAW resonator to improve mode confinement in the region of overlap of the top electrode, the piezoelectric layer, and the bottom electrode of a BAW resonator (commonly referred to as the active region).